A type of asynchronous motor known to the person skilled in the art comprises a stator having a first winding and a rotor having a second electrically-self-contained winding.
When the first winding is connected to a polyphase source of electric energy producing an AC supply current, this first winding generates a turning magnetic field in the vicinity of the rotor. When the rotor rotates at a frequency other than the frequency at which the turning magnetic field rotates, the flux of the turning magnetic field which passes through the rotor's second winding induces in this second winding an induced voltage generating therein an induced electric current. In this latter case, an electromagnetic force acts on the rotor as a result of the coupling between the turning magnetic field and the electric current induced in the second winding, which generates a torque on the motor's output shaft.
The slip S of an asynchronous motor can be defined by the following formula: EQU S=(FST-FRT)/FST
where:
FST=Frequency of rotation of the turning magnetic field generated by the supply current, PA1 FRT=Frequency of rotation of the rotor PA1 a stator winding arranged to produce a turning magnetic field at a stator frequency in response to an AC supply voltage applied to said stator winding and producing a supply current; PA1 a rotor comprising a rotor winding magnetically coupled to said stator winding, this rotor rotating at a rotor frequency in response to said turning magnetic field; the control device comprising electric supply means for producing said supply voltage and being characterized in that said electric supply means are arranged to produce said supply voltage with an amplitude determined by the value of a first control signal and with a frequency determined by the value of a second control signal, the first control signal being supplied to a first input of said electric supply means by first means for regulating the amplitude of the supply voltage, and the second control signal being supplied to a second input of said electric supply means by second means for regulating the frequency of the supply voltage, said first and second means being arranged so that, for each value of said stator frequency, said amplitude of the supply voltage is able to vary as a function of a first regulation signal between a minimum value and a maximum value defined for each value of said stator frequency, the set of said maximum values defining a limiting voltage curve, said supply voltage frequency being regulated such that for each value of said stator frequency, the difference of frequency of rotation between said stator frequency and said rotor frequency is maintained substantially constant as long as said supply voltage amplitude has a value situated below said limiting voltage curve.
For a given stator frequency FST and a given amplitude of the supply voltage, the supply current can be seen to increase when the slip S increases, and the torque increases when the slip varies between a zero value and a threshold value for which the maximum torque is reached. The range of values of the slip comprised between the zero value and the threshold value defines a range of operation of the motor for a given stator frequency FST and a given supply voltage.
Moreover, for a given stator frequency FST and a given slip S, an increase of the torque can be observed when the amplitude of the supply voltage increases as long as the motor has not reached magnetic saturation.
It will be noted that the saturation value of the magnetic flux through the rotor's winding defines, for a value of the stator frequency FST, a saturation value for the supply voltage amplitude, this saturation value being determined by the type of motor and its dimensions.
As the magnetic flux is substantially proportional to the supply voltage amplitude and inversely proportional to the supply voltage frequency (a whole number multiple of the stator frequency), the nominal operating curve of such an asynchronous motor is generally characterized, on a graph showing the supply voltage amplitude as a function of the supply voltage frequency, by a closely related curve corresponding to a substantially constant torque for a constant slip, over a first range of frequencies whose maximum value corresponds to the nominal maximum supply voltage amplitude. This closely related curve extends through a second range of frequencies following said first range of frequencies by a substantially constant curve situated at the level of the maximum nominal supply voltage amplitude.
For each supply voltage frequency and stator frequency FST respectively, the nominal supply curve sets a fixed and predetermined supply voltage amplitude. Hence, for a given stator frequency, the variation of the torque on the motor shaft is obtained by a variation of the slip S when the motor's operating point is situated on the nominal supply curve.
Generally, control of an asynchronous motor by means of an electronic device is arranged so that the operating point remains on the predetermined nominal supply curve. Such control of an asynchronous motor has several drawbacks. Firstly, in view of the fact that for a given supply frequency the motor always operates with a nominal voltage amplitude, the losses generated are relatively great whatever torque may be required, in particular as regarding the core losses. Secondly, variation of the slip S in order to vary the torque leads to non-optimal working conditions of the motor because the efficiency of an asynchronous motor depends on the value of the slip and hence on the difference between the stator frequency FST and rotor frequency FRT.
Hence, in most instances, the nominal amplitude of the voltage is too high for the required torque and the resulting slip is relatively low, which leads to a non-optimum working conditions of the motor. In other instances, when the required torque is relatively great, the nominal supply voltage amplitude, which is quite remote from the saturation amplitude, leads to a relatively great increase of the slip, which once again leads to non-optimal working conditions of the motor, the losses being in this case relatively great due to the large supply current required.